Hi, the confusion here was really caused by an error on my part. When designing the PCB I had connected PVDD up to the DVDD rail and at the time I hadn't even noticed that there was a P, I thought it said DVDD. I had however connected the PLL filter up to the 3.3v line and when I came to power on the board, nothing happened, except that the CLKOUT clock looked a mess. At this time I was still under the impression that PVDD should be connected to 1.8 volts, as should the PLL filter, but as I had connected it to 3.3 volts I thought it might have destroyed something. I fixed the error by cutting some traces and hooked PVDD up to the 3.3 volt line and everything jumped into action.